An assessment of differences between cloud effective particle radius retrievals for marine water clouds from three MODIS spectral bands

The Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product provides three separate 1 km resolution retrievals of cloud particle effective radii (r(e)), derived from 1.6, 2.1 and 3.7 mu m band observations. In this study, differences among the three size retrievals for maritime water clouds (designated as r(e,1.6) r(e,2.1) and r(e,3.7)) were systematically investigated through a series of case studies and global analyses. Substantial differences are found between r(e,3.7) and r(e,2.1) retrievals (Delta r(e,3.7-2.1)), with a strong dependence on cloud regime. The differences are typically small, within +/- 2 mu m, over relatively spatially homogeneous costal stratocumulus cloud regions. However, for trade wind cumulus regimes, r(e,3.7) was found to be substantially smaller than r(e,2.1), sometimes by more than 10 mu m. The correlation of Delta r(e,3.7-2.1) with key cloud parameters, including the cloud optical thickness (tau), r(e) and a cloud horizontal heterogeneity index (H-sigma) derived from 250m resolution MODIS 0.86 mu m band observations, were investigated using one month of MODIS Terra data. It was found that differences among the three r(e) retrievals for optically thin clouds (tau < 5) are highly variable, ranging from -15 mu m to 10 mu m, likely due to the large MODIS retrieval uncertainties when the cloud is thin. The Delta r(e),(3.7-2.1) exhibited a threshold-like dependence on both r(e,2.1) and H-sigma. The r(e,3.7) is found to agree reasonably well with re,(2.1) when r(e,2.1) is smaller than about 15 mu m, but becomes increasingly smaller than r(e,2.1) once r(e,2.1) exceeds this size. All three r(e) retrievals showed little dependence when cloud is relatively homogenous (H-sigma < 0.3 defined as standard deviation divided by the mean for the 250 m pixels within a 1 km pixel retrieval). However, for inhomogeneous clouds (H-sigma > 0.3), both r(e, 1.6) and r(e,2.1) were seen to increase quickly with H-sigma. On the other hand, r(e,3.7) statistics showed little dependence on H-sigma and remained relatively stable over the whole range of H-sigma values. Potential contributing causes to the substantial r(e,3.7) and r(e,2.1) differences are discussed. In particular, based on both 1-D and 3-D radiative transfer simulations, we have elucidated mechanisms by which cloud heterogeneity and 3-D radiative effects can cause large differences between r(e,3.7) and r(e,2.1) retrievals for highly inhomogeneous clouds. Our results suggest that the contrast in observed Delta r(e,3.7-2.1) between cloud regimes is correlated with increases in both cloud re and Hs. We also speculate that in some highly inhomogeneous drizzling clouds, vertical structure induced by drizzle and 3-D radiative effects might operate together to cause dramatic differences between r(e,3.7) and r(e,2.1) retrievals.